Not Applicable
Not Applicable
1. Technical Field
The invention relates to a microlithographic illumination system with an excimer laser having an emission wavelength, a beam expanding system, a light mixing system, and an illumination plane.
Such illumination systems are known in DUV microlithography with wavelengths of 248 nm, 193 mn, and 157 nm. Examples are described in European Patent Document EP 0 747 772.
In addition, the invention relates to a projection exposure device having an illumination system, a mask on a holding system, a projection objective, an exposure object on a support system, and a control and regulating system.
Use of a pseudo-depolarizer in such an illumination system is provided. The concept of pseudo-depolarizer makes it clear that these elements do not actually remove the polarization, but only bring about different polarization changes over the cross section of the pencil of rays, so that on the spatial average a preferred direction of polarization no longer exists.
2. Discussion of Relevant Art
Different pseudo-depolarizers are known from the literature:
The Hanle depolarizer, connected to a compensating wedge, is offered by the firm of Bernhard Halle Successors as an element for optical experiments.
German Patent Document DD 281 011 A1 describes a depolarizer consisting of a linear doubly refracting wedge and a circular doubly refracting second wedge for use in the UV region for measuring devices (spectrographs).
In Fuyun Xu, SPIE Vol. 1752 (1992), 307-310, various quartz depolarizers are described, including an arrangement of two wedges with crossed optical axes.
However, measures are not given for the superposition of the polarization distribution arising in these elementsxe2x80x94all polarization directions geometrically adjacentxe2x80x94and it is assumed there that an integration is effected by a detector extending two-dimensionally in the measuring devices.
It is advantageous to use unpolarized light in microlithography, in order to obtain a structure transfer, which is independent of direction. In microlithographic systems with wavelengths below 300 nm, however, linearly polarized excimer lasers are preferably used as the light source. These are available for the wavelengths of 248 nm, 193 nm and 167 nm.
The use of a xcex/4 plate, which produces circularly polarized light, is known for the production of light without a preferred direction of polarization. The requirements on the delay tolerance are very close there; for example, a delay error of xcex/100 still causes a residual polarization of 6%. The production of delay plates with such close tolerances is expensive at short wavelengths and hence correspondingly costly. Furthermore, a close tolerance can only be maintained over a small temperature range.
The invention has as its object the technically simple and cost-effective production of light without a preferred direction of polarization in the image plane of microlithographic illumination systems, or of projection illumination devices. According to the invention, the object is attained by a microlithographic illumination system comprising an excimer laser with an emission wavelength, a beam expanding system, a light mixing system, an illumination plane, and an optical element of doubly refracting material arranged in the cross section of a pencil of rays, wherein the thickness of the optical elements varies by a multiple of the wavelength over the cross section of the pencil of rays, and at least one light mixing system is arranged after the optical element.
A delay element with locally defined different delays (pseudo-depolarizer) is thus used instead of a delay plate having very close tolerances, and uniformly unpolarized light is produced in the whole illuminated field by light-mixing components of the subsequent optics. The variation of the thickness can then take place over an arbitrary, but not integral, multiple of the emission wavelength. The production of unpolarized light has the further advantage in contrast to circularly polarized light that succeeding components causing unintended polarization changes cannot cause any reversion of the polarization state into unfavorable elliptically polarized light. This is particularly so for very short wavelengths.
According to the invention, the depolarizing element is a wedge, particularly a Hanle depolarizer. A lens of doubly refracting material could also be used, for example. Providing a second optical element which raises the average refractive effect of the optical element is important in practice; it avoids in particular a bending of the optical axis and resulting complications in the construction of mountings.
Advantageously, the microlithographic illuminating system further comprises a further optical element of doubly refracting material with circular double refraction or with an axis of double refraction rotated relative to the optical element of doubly refracting material. The refractive compensation is connected to increased depolarization.
Advantageously two light mixing systems are provided on two mutually optically conjugate planes, and thus ensure mixing and depolarization at each place in the further beam path.
Advantageously, the microlithographic projection illuminating system comprises an illumination system, a mask on a holding system, a projection objective, an exposure object on a support system, and a control and regulating system, wherein the illuminating system comprises a microlithographic illumination system according to the invention as described above. The types of objectivesxe2x80x94purely refractive or catadioptricxe2x80x94are combined according to the invention described above.
Advantageously the catadioptric projection objective is of the axially symmetrical type, with central shadowing or of a modified Schupman achromat type.
Examples of refractive or catadioptric types of objectives as used in the invention are found in U.S. Pat. No. 5,260,832, German Patent DE 196 39 586, and U.S. Pat. No. 5,691,802.